Japanese Patent Application Publication No. 5-86913 (JP-A-5-86913) discloses an engine mounted with a hydraulically-driven variable valve timing mechanism for changing valve timing.
This variable valve timing mechanism is a mechanism capable of changing the timing of opening and closing at least one of intake and exhaust valves by advancing or retarding a rotational phase of at least one of intake and exhaust camshafts in relation to a rotational phase of a crankshaft.
The abovementioned hydraulically-driven variable valve timing mechanism has a controller that is constituted by a vane actuator installed on the front end side of the camshafts, and an oil control valve for adjusting the pressure of working oil supplied to an advance-side pressure chamber and a retard-side pressure chamber of the controller.
This engine might be provided with a supply passage for pumping up oil of an oil pan (oil storage) provided on a bottom part of the engine by using an oil pump and then supplying the oil to two systems of a cylinder block system and a cylinder head system. Note that the oil that lubricates the inside of the cylinder block and the inside of the cylinder head is returned to the oil pan, and thus the oil circulates in a closed loop within the engine.
The oil that is supplied to the cylinder block system is also supplied to a crank journal, oil jet, and the like. The oil that is supplied to the cylinder head system, on the other hand, is supplied in the form of working oil to the oil control valve of the variable valve timing mechanism described above.
Note that the oil that is supplied to the cylinder head system is supplied to a journal part positioned at a forefront end of a camshaft and to the oil control valve and each pressure chamber of the controller of the variable valve timing mechanism via oil passages provided within a forefront-end cam cap attached to a concave bearing of a cam housing that supports the forefront-end journal part. Note that the oil that returns from each pressure chamber of the controller is discharged into the cam housing through the oil passages.
As in the related art described above, in a case where part of the oil passages for supplying and recovering the oil to and from the controller and oil control valve of the variable valve timing mechanism is provided in the cam cap and camshaft attached to the forefront-end concave bearing of the cam housing, there is a possibility that the oil flowing through the oil passages leaks from a sliding contact surface where the forefront-end concave bearing of the cam housing and the cam cap come into sliding contact with the journal part positioned at the forefront end of the camshaft, into a timing chain cover.
This sliding contact surface is a part where the concave bearing and cam cap come into sliding contact with the camshaft, and thus needs a gap of appropriate thickness therebetween. For this reason, the oil leakage occurs easily.
Therefore, when the oil that leaks into the timing chain cover contacts with the timing chain, there is a risk of friction loss that is caused by rotation resistance of a timing chain. Especially when the oil leaking from the sliding contact surface into the timing chain in spite of the system that lubricates the timing chain using the oil, excessive amount of oil is supplied to the timing chain, resulting in unexpected grow of friction loss.
Since it is desired to alleviate the friction loss as much as possible in view of improving engine efficiency and fuel consumption performance, there is room for improvement.
Note that even in an engine that is not mounted with the above variable valve timing mechanism, oil leaks from a sliding contact surface where a forefront-end journal part of the camshaft comes into sliding contact with the concave bearing of the cam housing and with the cam cap.